A critical vulnerability patch is now enforced by Microsoft for Remote Desktop Protocol. This affects RemoteApp, Azure hosted Remote Desktop, and standard remote desktop. Basically both the server and the client PC must be up to date in order to establish a remote connection. If either instance is not patched, you may see the CredSSP encryption error:

To check for updates in Windows 10 open Start > Settings > Update and Security and click Check for updates

In earlier versions of Windows open Control Panel > Windows Update > Check for updates